Is this just some marketing change that hasn't been updated everywhere, and habituation to an old name? Because when I search now, the only actual Airfoil Pro product I find is some sort of bicycle attachment.

Airfoil Pro is the name of the rewritten plugin that we shipped with Indigo 7. It uses a new(ish) network-based API so that it can control Airfoil instances on the network (Mac or Windows). The old Airfoil plugin used AppleScript and was therefore limited to only the instance running on the Indigo Server Mac.